The AutoMailMerge can send emails with PDF attachments to the recipients listed in the spreadsheet or any other data file.
To configure, you must give AutoMailMerge an SMTP server it can use for sending the e-mails.
Many organizations have an internal SMTP server you can use and hosting companies often provide one as well.
Widely popular Gmail provides an SMTP server that you can use to send up to 2,000 messages a day.
This tutorials tells you how to set up Gmail as an SMTP server when your Google account is
configured with 2-factor authentication.

For increased security your Google account may be using 2-factor authentication,
also known as 2-step verification. When enabled, you must log in to Gmail with both your password and
your phone. This prevents hackers who get your password from getting into your account,
since they won’t get the security code sent to your phone.

This method requires App Passwords to be used to send emails from the plug-in.
An App Password is a 16-digit passcode that gives a non-Google app or device permission to access your
Google Account. App Passwords can only be used with accounts that have 2-Step Verification turned on.

Step 1 - Enable 2-Factor Authentication in Google Account

Open a web browser and login into your Google account.
Click on "Security" menu (it can be located on the left or at the top of the webpage).

Step 2 - Select "2-Step Verification"

If "2-Step Verification" is not turned on, then click on the it to enabled it.